(1) Field of the Invention
This invention relates to an optical system flatness measuring apparatus capable of measuring the flatness of an object with hyperaccuracy and automatically.
(2) Description of the Prior Art
There has been a tendency to require an extremely precise flatness of 0.05 .mu.m or less for a sliding surface of a magnetic head at a magnetic tape recording apparatus. Various methods of utilizing the light interference have been used for non-contact measurement of the flatness, but there has never been found any example in practical use because many problems are created in the precision of components, hyperfine adjustment, and maintenance of the measurement accuracy, from the viewpoint of carrying out easy and stable measurement with the above accuracy.
The inventor has proposed an optical flatness measuring method (in Japanese Patent Laid-Open No. Sho 54-159258) aiming mainly at the automation of measurement. The principle of the above is utilized for the present invention, whereby explanation at first will be given on the principle of the former invention.
For example, a Fizeau optical interferometer, as shown in FIG. 1, is used to obtain interference fringes 1 caused by the plane of an object to be detected and the reference plane, such as an optical flat, the bright fringes being shown by solid lines and dark fringes by broken lines in the drawing. One-dimensional image sensor 34 matched lengthwise thereof with the directions of widths of interference fringes 1 is used to catch the pattern thereof at three positions a, b and c, so that the positions Pa, Pb and Pc widthwise of the fringes at the predetermined positions a, b and c of the bright (or dark) fringes and a pitch P of the fringe are detected by an output of image sensor 34, thereby computing a flatness F by the following equations: ##EQU1## where .lambda.: interference light source wavelength, and
n: interference order (herein n=1),
and ##EQU2##
The inventor has used the above method to commence manufacture of the apparatus measurable of the flatness of an object with accuracy of 0.05 .mu.m or less as abovementioned, in which the accuracy for processing various components and for assembly thereof and of optical parts, is required to be extremely high. Even when such accuracy is satisfied, a desired measurement accuracy, under the influence of external vibrations, cannot be achieved, whereby the apparatus has been extremely difficult for practical use. In order to thoroughly solve the above problems, the present invention has been designed.